Sealing system for in-ground barrier

ABSTRACT

The joints between pile-driven sheet metal barrier elements (12, 14) have edge forms which interlock so as to form cavities (20, 21). The cavities extend from top to bottom of the barrier, and allow a flushing hose to be inserted to the foot of the barrier. Sealant is injected when the cavity is flushed clean. Two cavities are provided side by side at each joint. The cavities are independently sealed for extra reliability.

This invention relates to in-ground barriers, of the kind that comprisepile-driven elements of sheet material, and the like. Such barriers areoften required to be watertight, or at least to be leak resistant.

BACKGROUND TO THE INVENTION

Patent publication GB-2228760 (VALES, published 05 Sept 1990) shows anarrangement of the interlocking edge forms of the elements of a barrier,the illustrated arrangement being highly effective to resist leakage.The present invention may be regarded as an improvement to that type ofinterlocking edge-form arrangement.

In VALES, one of the key aspects is that a cavity is created in thepotential leak path that exists between two interlocking elements. Thecavity is defined by the profiles of the interlocking edge forms. Thecavity is large enough that a hose pipe may be passed down the cavity,and water then may be flushed through the cavity. When the cavity isflushed out, an injecting or dispensing pipe may be inserted into thecavity, and a sealant and/or adhesive may be injected into the cavity.

The present invention is aimed at improving the reliability with whichthe sealed junctions of this general type can be regarded as leak proof.

GENERAL FEATURES OF THE INVENTION

The invention consists in a leak-resistant barrier. Each element of thebarrier has a senior edge form which interlocks with, and is pile-drivenor otherwise inserted ahead of, a junior edge form of the next adjacentelement.

In the invention, the configurations of the interlocking pairs of seniorand junior edge forms are such that the configurations thereof jointlyform the circumferences of two enclosed cavities.

In respect of each one of the two enclosed cavities, a portion of thecircumference of the enclosed cavity is constituted by a portion of thesenior edge form, and another portion of the circumference of that sameenclosed cavity is constituted by a portion of the interlocking junioredge form.

In respect of both cavities, the portion of the circumference of thecavity constituted by the junior edge form preferably is the majorportion of that circumference.

The barrier includes means for keeping the said enclosed cavities clearof dirt and debris when the barrier is installed. Preferably, this takesthe form of scrapers attached to the foot of the junior edge form, whichact to deflect the dirt and debris laterally away from the cavities asthe junior edge form is driven onto the senior.

In respect of each one of the two enclosed cavities, the edge forms areso shaped that each enclosed cavity includes a respective clear, openspace, which is defined by and inscribed wholly within the enclosedcavity, the inscribed circle being clear and open in that no portion ofthe material of either of the elements encroaches into the saidinscribed circle. This clear, open space permits the insertion into thecavity of the flushing hose and/or a sealant injecting tube. The cavityis open from top to bottom of the barrier, so that the hose can beinserted from the surface all the way down to the bottom of the barrier.The inscribed circle is preferably of at least 18 mm diameter.

As mentioned, the interlocking senior and junior edge forms jointly formthe whole circumferences of the cavities, with the result that potentialleakpaths are created between the edge forms. It is arranged that eachand every leakpath starting from in front of the barrier and finishingbehind the barrier is in communication with the said two enclosedcavities, and in fact the interlocking edge forms are so arranged thatall the potential leakpaths traverse through both the cavities, inseries.

The interlocking adjacent elements are so arranged as to include aninterlocking dovetail connection, being a connection which is effectiveto prevent lateral displacement of the senior edge form relative to theinterlocking junior edge form and to prevent consequent distortion ofthe cavity, the prevented displacement being displacement of suchmagnitude as to cause such lateral distortion of the cavity wherein theopen, clear inscribed circle preferably is less than 18 mm diameter. Inthe invention, the cavities are held rigidly apart and open at alllocations of the height of the barrier; if the edge forms were allowedto approach each other, the resulting mismatch between the edge formsmight be such that the cavities would no longer be sufficiently wideopen as to receive the flushing hose.

Preferably, the interlocking edge forms are of constant configurationfrom top to bottom of the elements, whereby, when the barrier isinstalled in the ground, each of the two cavities is clear, open, andaccessible from top to bottom of the barrier.

THE PRIOR ART

A previous proposal for a barrier which has two cavities at a joint isshown in U.S. Pat. No. 3,302,412 (HUNSUCKER, published 07 Feb 1967). Thepresent invention differs from Hunsucker in that in Hunsucker, bothcavities are filled with adhesive from the one filling point: sealantmaterial flows out of one cavity into the other. In the invention,sealant is inserted, from the surface, into the two cavitiesindependently. In the present invention, also the flushing hose can bepassed from top to bottom of the cavities independently.

Because of these differences, the two cavities can be regarded as beingindependent as regards sealing performance and efficiency. The leakresistance of the combined double cavity joint, with the arrangement ofthe invention, is the product of the leak resistances of the twoindividual cavities; in Hunsucker, the combined leak resistance is nomore than the sum of the individual leak resistances.

Preferably, both cavities are so formed that the junior element formsthe major portion of the circumference of the cavity. In respect of bothcavities, where scrapers are provided at the foot of the junior elementto clean out the cavities as the junior element is driven in, twoadvantages arise because the junior portion of the circumference islarge: first, the scraper, which sometimes can be vulnerable to beingdamaged during driving, is attached to the junior edge form over a largearea; and second, since the senior edge form portion of thecircumference is small, it is easy for the dirt and debris to be ejectedfrom the cavity.

In Hunsucker, the cavities are symmetrical: in the invention, thecavities are not symmetrical in that, in respect of both cavities,preferably the junior edge form supplies the major portion of thecircumference of the cavity.

As regards the dovetail connection, it is preferred that the elements beso arranged as to include no redundant dovetail connections. If theelements were to be too well located with respect to each other, forexample if two or more dovetail connections were provided at the joint,the dovetail connections might "fight" each other. The one dovetailconnection rigidly prevents the edge forms from approaching orseparating, both in the front/back sense and in the left/right sense,but sheet piling elements inevitably cannot be made to high degrees ofdimensional tolerance, and slight inaccuracies must be expected. Apartfrom the one dovetail connection, therefore, the fit of the elements oneach other should be quite loose.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

By way of further explanation of the invention, exemplary embodiments ofthe invention will now be described with reference to the accompanyingdrawings, in which:

FIG. 1 is a plan view in cross-section of a portion of an in-groundbarrier, showing a particular configuration of pile-driven sheet metalbarrier elements;

FIG. 2 is a corresponding view of a second barrier;

FIG. 3 is a side elevation of a barrier that has been installed in theground, and in which the operations of cleaning out the cavities andinserting sealant are being conducted;

FIG. 4 is a view corresponding to FIG. 1 of a third barrier;

FIG. 5 is a pictorial view from below of the pair of interlockingelements shown in FIG. 2.

The items shown in the accompanying drawings and described below areexamples which embody the invention. It should be noted that the scopeof the invention is defined by the accompanying claims, and notnecessarily by specific features of exemplary embodiments.

FIG. 1 is a plan of an area of ground, viewed from above, into which hasbeen driven, by pile-driving, two sheet metal elements 12,14. Theelements 12,14 are joined at a junction 16, which is aimed at beingwatertight.

The element 12 is formed with a right-hand edge-form 18, and the element14 is formed with a left-hand edge-form 19, the two edge-forms being ininterlocking engagement. The edge-forms 18,19 can be assembled onlyvertically. Once assembled together, the elements can only be separatedby relative sliding of the edge forms vertically. The edge forms, wheninterlocked, locate and hold the two elements, in the lateral sense,very firmly relative to each other.

The whole barrier is made up of elements joined together by means ofjunctions like junction 16.

The interlocking edge forms 18,19 define a pair of cavities 20,21. Eachof these cavities is roughly circular in outline, though not exactly so,as may be seen. Each cavity is large enough that a clear circle 23 ofdiameter about 2 cm can be inscribed inside the plan view of the cavity.

The size of the inscribed circle, which defines the clear cavity, shouldbe such that a flushing hose can be easily passed down the cavity fromtop to bottom. The cavity should also be large enough that, when theflushing hose is in place, flushing water from the hose can pass up thecavity, around the hose, and out at the surface. The type of hose thatwould be contemplated for use with a practical barrier would be ofstandard half-inch size: the practical minimum diameter of the inscribedcircle for use with such a flushing hose is about 18 mm.

The elements include a dovetail connection 23. By means of the dovetailconnection, when the edge-forms are interlocked, there can besubstantially no lateral movement of the edge-forms relative to eachother. The edge-forms should be so shaped as to prevent the edge-formsfrom moving laterally--from approaching each other, forinstance--because such approach would encroach into the inscribedcircles 24.

On the other hand, the edge-forms should not be so tight to each otherthat they interfere: when the elements are being pile-driven, suchtightness can cause high friction forces to develop at the points ofcontact, which can even be sufficient to heat the metal to the point offusing.

The fact that there are two cavities is important, as will become clear.

FIG. 2 illustrates another arrangement of interlocking edge-forms inwhich two cavities are provided. In FIG. 1, the edge-forms 18,19 werecreated by hot-rolling, in which the metal is upset, ie thecross-sectional thickness of the metal is changed and deformed. In FIG.2, the edge forms 32,34 were produced by cold-rolling, in which themetal can be bent, but substantially cannot be deformed as to itsthickness.

In FIG. 2, two cavities 36,38 are provided. One cavity 38 is formed by aloop in the edge forms, whereas the second cavity 36 is formed bywelding on an extra piece 40 of metal. It may be noted that the weldingcan consist of intermittent short tack-welds, there being no need forthe welded seam itself to be watertight. Although welding is labourintensive, cold rolling may be preferred as an inexpensive process forshort production runs.

As described in GB-2228760, a scraper is provided at the foot of theelements, and the scraper acts to clear dirt and debris out of thecavity. With the presence of the scraper, when the pile elements arefully driven into the ground, each cavity may be expected to be opendown to the bottom of the barrier. That is to say, the cavities can beexpected to be free from large pebbles.

However, the cavities cannot be expected to be dirt free. If any dirtshould be present in the cavity, the sealant might not penetrate intoall the crannies of the potential leak paths between the elements. Also,even with the scraper present, larger debris sometimes does collectinside the cavity.

Generally, if the soil into which the barrier is being inserted ishomogeneous sand and gravel, the cavities will be free of dirt anddebris, and the sealant will be complete and the joint leakproof.

But the ground into which the barrier is installed may be a soil mixtureof gravel and clay. Or the ground may include dried out, cracked clay.It is possible for such cohesive clay material to enter the cavity innot insignificant quantities. Once in, it is possible for a clump of thecohesive material to coalesce, and to adhere to the inside walls of thecavity.

It is even possible for the walls of the cavity to be lined with stickyclay, to the extent that the clay defines a tube, down which theflushing hose may pass in that case, it might be possible for theengineer to determine that there is no obstruction in the cavity, andyet the sealant injected into the cavity cannot actually reach and touchthe inside walls of the cavity.

Therefore, even with the cavity/scraper/flush system as described inGB-2228760, it is still possible, especially in certain kinds of soils,that the injected sealant might not be fully effective to produce aleak-proof cavity.

On the other hand, the number of cavities that do leak can be expectedto be small. The provision of two independently-sealed cavities at thesame junction, then, as a matter of statistical chance, virtuallyeliminates the chance of a leak.

For example, if the cavity/scraper/flush system can be expected to leakat the rate of one in a thousand joints (which would be of someconcern), if two independently-sealed cavities are provided the expectedleak rate goes to one in a million joints (which can be ignored).

It should be emphasized that even when the flushing hose will passfreely right down to the foot of the cavity, with some kinds of soilthat is still not enough to be sure that there might not be some dirtstill adhering to the walls inside the cavity.

Even more so, the joint cannot be expected to be fully sealed if thecavity is in fact found to contain a detectable obstruction of somekind, whereby the flushing hose cannot pass right down the cavity. It isrecognised that, even with all the precautions taken to keep the cavityclear, it still can happen that a pebble etc can become lodged in thecavity.

If this does happen, it will invariably not be discovered until thebarrier is fully inserted. It is usually impossible to take out just asingle element of the barrier, clean it, and then put that element back.Usually, the whole barrier has to be taken out if it is desired to clearan obstruction from just one cavity.

This is of course an horrendous penalty, and the temptation on the partof the contractor is often to simply inject the sealant into the cavityas far down as he can, and keep quiet about the obstruction. It may beyears before it is discovered that the joint is in fact leaking,especially if leaking were not suspected.

With the provision of the two cavities per joint, the chance of the twocavities at the same joint being both blocked by pebbles can be ignored.Both cavities can be made to serve to seal the joint, independently ofthe other. This source of potential leakage therefore is virtuallyeliminated by the provision of double cavities.

Another reason for preferring double cavities is that different sealantsand/or adhesives can be placed in the two cavities. For instance, aparticularly active sealant or adhesive might be very good for sealingcertain kinds of contamination, but might itself put toxic traces intowater. In that case, the engineer might elect to inject the activematerial into one cavity, whilst a more inert passive material could beinserted into the other cavity.

It may be preferred in some cases to use a two-componentadhesive/sealant (many of the foaming sealants are of the two-componenttype, for example). The double cavity arrangement permits the twocomponents not to be pre-mixed, but to be kept separate until thecomponents are actually in place. (Once a two-component sealant has beenmixed, it must be used immediately, which can pose some operationalrestrictions.)

It will be noted that the double cavity arrangement actually providesnot only the two cavities, but also provides a path between the twocavities. This path is more or less narrow and tortuous. Some kinds ofadhesive are bulk sensitive, in that they will set or cure differentlyin a narrow pathway, as compared with their activity when in a largebulk. The double cavities provides both large bulk areas and narrowtortuous areas, so that both kinds of properties may be catered for. Thekinds of sealants and/or adhesives inserted into the cavities mustusually be the kinds that will set and cure while immersed in water, andthose kinds can be especially sensitive to the tortuous-pathway/in-bulkdifference.

In the FIG. 1 arrangement, the elements 12, 14 are so firmly held by theedge forms 18, 19 as to be extremely resistant to articulation betweenthe elements. In fact, in the arrangement of FIG. 1, it will be seen,the provision of the two cavities allows the furthermost contact points30, at which the two edge forms engage each other, to be very wellspaced out, which makes the joint highly resistant to articulation.

This resistance to articulation is an advantage in ensuring that theelements do not wander out of line below ground. If such resistance toarticulation were not provided, the fully inserted elements might, ifthe ground were uneven, be rippled and almost wavy. So long as the jointis still leak proof that perhaps does not matter, but the effect is notself correcting and later driven elements pick up all the out of lineerrors of the already inserted elements, and the cumulativemisalignments can be troublesome. An arrangement of the joint thatpermits articulation therefore is mainly suitable for plain sands andgravels, where there are unlikely to be non-homogeneities of sufficientsubstance to drive the element off line.

If boulders etc are embedded in clay, the high articulation-resistanceof the FIG. 1 arrangement, by contrast will often permit the drivenelement actually to fracture a boulder that lies in the line of theelement, rather than be deflected aside by it.

It may be noted that this high articulation resistance, which comes fromhaving the contact points 30 between the edge forms spaced well apart,arises virtually without cost when the double cavities are provided, asshown in FIG. 1. On the other hand, the double cavities can be providedwhile still allowing the joint to articulate, if the nature of theground, the layout of the barrier, etc permit, or dictate, that that ispreferred.

FIG. 3 shows a barrier in which the elements have been fully driven intoposition. A hose 43 is passed down right to the bottom of one of thecavities, and the water supply is turned on. The water from the hosepasses up through the cavity, around the hose, flushing out such dirtand debris as may be present, until the water runs clear. Next, adispensing pipe 45 is inserted to the bottom of the cavity, andsealant/adhesive is injected though the pipe as required. The dispensingpipe is progressively withdrawn up the cavity as the sealant is injectedinto the cavity. These two operations are carried out in respect of bothcavities at each junction.

As mentioned, at the foot of the edge forms is attached a scraper. Theelements are driven in sequence, and the first-driven element of anadjacent pair is termed the senior element, and the last-to-driveelement is termed the junior element. The final outline of the cavity isdefined in part by the edge form of the metal of the senior element andin part by the edge form of the metal of the junior element.

The scraper is attached at the foot of the junior element, and itsfunction is to pass down around the shape of the edge form of the seniorelement as the elements are driven together, and to deflect aside anydirt, pebbles, etc as may be present in what will be the cavity. Whentwo cavities are provided, each is furnished with a scraper.

FIG. 2 shows the scrapers in plan view, looking down from above theelements. Both scrapers 47,49 are attached to the foot of the junioredge form 34. The drawing shows the extent to which the scrapers aresupported by the shape of the junior element. A scraper of courseencounters serious abusive forces if it should happen to encounter atroublesome obstruction, and its function is to remove that obstruction.Therefore, it is preferred that the scraper should be well-supported onand by the foot of the junior edge form: the scraper should not, forexample, be welded at a single narrow point, and cantilever out for alarge distance.

It is preferred that the junior edge form should constitute a majorproportion of the outline of the whole cavity, for two reasons:

first it is preferred that the junior-formed portion of the cavity belarge, because there is then ample material in the junior edge-formwhich is available for attaching the scraper without much cantilevering;

and secondly it is preferred that the senior-formed portion of thecavity be small, because the senior edge-form is then wide open and caneasily release, ie will not entrap, any debris materials that may bedeflected by the scraper.

Thus, it is preferred, from this standpoint, that the profiles of thejunior and senior edge forms not be the same as each other but that, asin FIGS. 1 and 2, the junior profile constitute the major share of thewhole cavity profile, in respect of both cavities.

The scraper should lie at an angle, whereby it can deflect the materialsit encounters sideways out of the cavity, rather than simply compressingthe materials underneath the scraper.

FIG. 5 shows the two scrapers 47,49 disposed one either side of thesenior element. The two scrapers are arranged in V-formation, wherebythe forces due to debris being deflected are substantially equalised,and the tendency of the barrier to be deflected laterally is minimised.FIG. 5 shows the foot of the elements of FIG. 2. However, theV-formation is not essential, and the scrapers for the other jointsillustrated will not have the symmetrical V-formation, as may beunderstood from a perusal of the plan views thereof.

The preferred orientation for the angle of the scraper is that thescraper should be angled so that points A of the FIG. 2 form areuppermost. This means cutting the foot of the junior edge form at theappropriate angle. The scraper is a small piece of sheet metal welded tothe angled surface of the foot of the junior edge-form.

For two cavities, a one-piece scraper covering both cavities, or twoseparate scrapers, may be provided. The FIG. 1 arrangement lends itselfto a one-piece scraper. So does the FIG. 4 arrangement. Of course, aone-piece scraper is inappropriate for the arrangement of FIGS. 2,5.

In fact, the preferred scraper angle for the two cavities need not bethe same. In this case, two angled faces may be cut at the foot of thejunior edge form, each lying at a different angle. Alternatively,especially when a one-piece scraper is being used, a single compromiseorientation of the angle may be used. The orientation of the angleshould be such that the debris is deflected towards a portion of thesenior edge form that is wide open, and yet the designer should takecare not to create undercuts or exposed promontories in the foot of theedge form, since of course it is the foot of the element that receivesthe most abusive forces and stresses during driving.

As mentioned, the dovetail connection at the joint should not includeredundant connections. Thus, in FIG. 2, the front/rear location of theedge forms relative to each other is accomplished by means of the fit63, as shown. Similarly, the left/right location of the edge forms isaccomplished by the fit 65. It will be observed that there are no othertight connections or engagements between the edge forms that would"fight" these locating-fits. The fits 63 and 65 should be the tightestengagements between the elements: all other possible touchingengagements between the elements should be more free than thelocating-fits 63,65.

Although there are two cavities, there is only one dovetail connection.(The dovetail connection comprises a location-fit in the front/rearsense, and a location-fit in the left/right sense.)

The dovetail connection may be provided by specific components providedfor that purpose, or the dovetail connection may be integrated intothose fits between the elements that comprise the cavities. Thepreference for no redundancy in the location-fits applies even where thelocation fits include portions of the circumference of the cavity. Thus,for example, in a case where the dovetail connection were constituted bythe fit of the junior portion of one of the cavities over the seniorportion of that cavity, it would be important that the junior and seniorportion of the other cavity were loose upon each other.

FIG. 4 shows an arrangement where the dovetail connection is created bythe welded-on shapes, as shown, being a channel 69 and an angle 70. Itmay be noted that the welded-on shapes need not be continuous in theFIG. 4 arrangement: leakage of sealant between the cavities would notmatter. (Leakage of water of course should not be permitted.) Where theshapes can be made in short lengths, manufacturing savings may bepossible.

It was noted above that in FIG. 2 the welded-on angle 40 need not becontinuously welded, since the welded connection is not required itselfto be watertight. This is a matter of geometric placing, however, and itis not ruled out, in the invention, that a welded connection might be soplaced that the welded connection must be watertight.

The thickness of pile-driven sheets of course varies. The invention isintended for use with sheets in the range from about 6 or 8 mm to 10 or12 min. Such sheets are usually made of metal, ie steel; it is knownhowever to make barrier sheets from plastic material, though in thiscase the sheets are generally not driven directly. Rather, a metal sheetis driven (ie hammered) in order to create a receptacle for the plasticsheet. The distinctive feature of the invention is the double cavities,which can be embodied in a plastic barrier.

Particularly in the case of the plastic barriers, the adhesive orsealant can be in the form of a cylindrical bead of water-curingmaterial, of perhaps 6 mm diameter. The invention lends itself to thiskind of sealant, in that the beads can be inserted from the surface downinto the cleared cavities with little chance of being impeded.

We claim:
 1. A barrier, which comprises a series of elements of sheetmaterial, wherein, upon installation of the barrier into the ground:eachelement has a senior edge form which interlocks with a junior edge formof a next adjacent element; the configurations of interlocking pairs ofsenior and junior edge forms are such that the configurations thereofjointly form the circumferences of two enclosed cavities; in respect ofeach of the two cavities, a portion of the circumference of the enclosedcavity is constituted by a portion of the senior edge form, and anotherportion of the circumference of that same enclosed cavity is constitutedby a portion of the interlocking junior edge form; in respect of eachone of the two enclosed cavities, the edge forms are so shaped that eachenclosed cavity includes a respective open circular space, which isdefined by and inscribed wholly within the enclosed cavity, theinscribed circle being clear and open in that no portion of the materialof either of the elements encroaches into the said inscribed circle; theinterlocking senior and junior edge forms are so arranged that potentialleakpaths to the leakage of water through the barrier are createdtherebetween, and are so arranged that each and every leakpath startingfrom in front of the barrier and finishing behind the barrier is incommunication with the said two enclosed cavities; the interlocking edgeforms are so arranged that all the potential leakpaths traverse throughboth the cavities, in series; characterised in that: the elements are soarranged as to include an interlocking dove-tail connection, being aconnection which is effective to prevent displacement of the senior edgeform in the horizontal sense relative to the junior edge form, andthereby to prevent consequent distortion of the cavities, theinterlocking edge forms being so arranged that the two cavities, eachindependently of the other, are clear, open, and directly accessiblefrom the top of the barrier, from top to bottom of the barrier; each ofthe two cavities is open and accessible to the extent that a hose pipecan be passed from the top of the barrier right down inside the cavityto the bottom of the barrier; and the two cavities are sealablyindependent of each other to the extent that one of the cavities can besealed, and the potential leakpaths communicating with that cavity canbe sealed, from top to bottom, independently of whether the other cavityis sealed.
 2. Barrier of claim 1, wherein:the arrangement of theinterlocking edge forms is such that the potential leakpaths arecomprised by contacting interfaces between the edge-forms; thearrangement of the contacting interfaces is such that the potentialleakpaths are tight and tortuous; the potential leakpaths are tight andtortuous enough to substantially contain a sealant within the cavity. 3.Barrier of claim 2, wherein the arrangement of the interlocking edgeforms is such as to define three of the said tight and tortuouspathways, the first being a pathway from one of the cavities to thefront of the barrier, the second being a pathway from the other of thecavities to the rear of the barrier, and the third being a pathwaybetween the two cavities.
 4. Barrier of claim 1, wherein the inscribedcircle is clear and open over a diameter of at least 18 mm.
 5. Barrierof claim 1, wherein in respect of both cavities, the portion of thecircumference of the cavity constituted by the junior edge form is themajor portion of that circumference, and the portion of thecircumference of the cavity constituted by the senior edge form is theminor portion of that circumference;
 6. Barrier of claim 1, wherein thematerial of the elements is steel.
 7. Barrier of claim 6, wherein themetal is of a thickness between 6 mm and 12 min.
 8. Barrier of claim 7,wherein the elements of the barrier were inserted into the ground bybeing pile-driven.
 9. Barrier of claim 6, wherein the material of theelement is of the same thickness throughout the element.
 10. Barrier ofclaim 9, wherein the junior edge form includes a loop and shape that iswelded onto the element, and wherein the loop comprises the majorportion of one of the cavities, and the welded-on shape comprises themajor portion of the other of the cavities.
 11. Barrier of claim 10,wherein the welded-on shape extends over the whole height of thebarrier.
 12. Barrier of claim 1, wherein:the means for keeping thecavities clear of dirt and debris comprise scrapers; the scrapers areattached rigidly and robustly to the elements, at the foot of theelements, beneath the junior edge form thereof; each scraper is of suchshape, and is so positioned beneath the junior edge form that, in aprojected plan view of the barrier, the scraper substantially fullyoccupies the shape of a respective one of the cavities; each cavity isopen and clear above the respective scraper, to the extent that scrapercan pass all the way down the senior edge form from top to bottom of thebarrier.
 13. Barrier of claim 12, wherein the arrangement is such thatthe scrapers are disposed either side of the senior element, and thescrapers are so orientated as to lie in a V-formation.
 14. Barrier ofclaim 1, wherein:the dovetail connection comprises means for tightlylocating the interlocking edge forms together in a front/rear sense andin a left/right sense; and the arrangement of the interlocking edgeforms is such that no other engagement between the edge forms locatesthe edge forms to each other more tightly than the said dovetailconnection.
 15. Barrier of claim 1, wherein the barrier is a barrierthat has been arranged according to the following procedure, in respectof each cavity:after installation of the elements in the ground, a hosewas installed in the cavity, from the top of the barrier, right down tothe bottom of the barrier, and water was passed through the hose in amanner that was effective to flush out dirt and debris from the cavity;after flushing out the cavity and withdrawing the hose, a dispensingpipe was installed in the cavity, from the top of the barrier, rightdown to the bottom of the barrier, sealant was injected through thepipe, and the pipe was progressively withdrawn up the cavity as thesealant was injected into the cavity.
 16. A barrier, which comprises aseries of elements of sheet metal, wherein, upon installation of thebarrier into the ground:each element has a senior edge form whichinterlocks with a junior edge form of a next adjacent element; theconfigurations of interlocking pairs of senior and junior edge forms aresuch that the configurations thereof jointly form the circumferences oftwo enclosed cavities; in respect of each of the two cavities, a portionof the circumference of the enclosed cavity is constituted by a portionof the senior edge form, and another portion of the circumference ofthat same enclosed cavity is constituted by a portion of theinterlocking junior edge form; in respect of each one of the twoenclosed cavities, the edge forms are so shaped that each enclosedcavity includes a respective open circular space, which is defined byand inscribed wholly within the enclosed cavity, the inscribed circlebeing clear and open in that no portion of the material of either of theelements encroaches into the said inscribed circle; the inscribed circleof each cavity being clear and open over a diameter of at least 18 mm;the junior edge form includes a loop and shape that is welded onto theelement, and wherein the loop comprises the major portion of one of thecavities, and the welded-on shape comprises the major portion of theother of the cavities; the interlocking senior and junior edge forms areso arranged that potential leakpaths to the leakage of water through thebarrier are created therebetween, and are so arranged that each andevery leakpath starting from in front of the barrier and finishingbehind the barrier is in communication with the said two enclosedcavities; the interlocking edge forms are so arranged that all thepotential leakpaths traverse through both the cavities, in series; eachof the two cavities is open and accessible to the extent that a hosepipe can be passed from the top of the barrier right down inside thecavity to the bottom of the barrier; and the two cavities are sealablyindependent of each other to the extent that one of the cavities can besealed, and the potential leakpaths communicating with that cavity canbe sealed, from top to bottom, independently of whether the other cavityis sealed.